That joke about the note in a clairvoyant's window reading "Closed due to unforeseen circumstances" springs to mind every time a conversation arises about the future of the Williams sisters. Serena turned all logic on its head in Melbourne this year when, ranked No81, and having hardly played any tennis of note in the previous year, she tore the opposition into shreds for her eighth slam title. Similarly Venus suddenly rediscovered her champion's pedigree two years ago to win her third Wimbledon, 3½ years after her previous major.

So when Venus was beaten 6-4, 4-6, 6-1 by Serbia's fast-rising Jelena Jankovic in the third round of the French Open yesterday, the talk was less of failure but rather what the American might achieve when she steps through the gates of the All England Club in just over three weeks' time.

Such is the power of the sisters, and such is the reluctance to ever write them off because they make fools of everybody. They may be accused of being part-timers, of using the WTA Tour strictly for their own ends, but like it or not they have always been champions.

"I got a little tired at the end. It's tough on clay," said Williams. "I'll keep working hard. Grass is definitely a good surface for me. In my mind I want to play fast. It's not that I can't play slow but at Wimbledon it's easier for me." She then went on to suggest the reason she faded so badly against Jankovic, the No4 seed, in the third set was that she had been overdoing her training.

"On my days off," she said, "I think I've just been training too hard. I'll have the court for one hour, and then no one comes, and I'll hit for two. So I've got to scale back, and not be an over-achiever. I know that."

This seems to make no sense at all. But then it is always unwise to equate what the sisters say after a match with what actually happened on court, or what has been happening in their lives. This was Jankovic's third successive victory over Venus, including a third-round win at Wimbledon last year, albeit when the American had wrist problems, and it underlined her rapid improvement over the last 12 months.

The one major doubt on this occasion was whether she would be able to cope temperamentally with meeting one of the Williams at such an early stage of the tournament. In Melbourne she lost to Serena in the fourth round.

Jankovic, having lost 10 of her opening 11 matches last year, was on the point of quitting before 2006's Italian Open when her form and fortunes turned dramatically for the better: "I'm a really bad loser, and I lost so many times that I no longer wanted to play or practise. I lost the motivation and the drive. If you are going to do your job, then you have to do it 100%." Such an argument has never unduly bothered either of the Williams sisters, but then they have always been the exceptions.

One of Jankovic's most endearing qualities, over and above her flowing shots, anticipation and fleetness of foot, is her ability to laugh at herself. She also shared the fun here with her entourage in the players' box: "I have really positive people supporting me and that helps me to release the tension. Before we go on court, all the other players are so quiet with their teams but I like to laugh and joke. People look over at us and probably think we are a bunch of clowns."

To date the 22-year-old Serb's best run in a grand slam tournament came at last year's US Open where she lost to Belgium's Justine Henin, the French Open champion for the past two years, in the semi-final.

Having won two titles on clay this year, notably the Italian Open a fortnight ago, Jankovic emerged as one of the key contenders, and yesterday's victory over Venus Williams emphasised her growing maturity and burgeoning self-belief. Jankovic served with great accuracy and on the occasions when she ventured to the net volleyed with flair and precision, suggesting that her already fluid game may develop to greater heights.